The present invention relates to a process for the separation of catalyst-free working solution from working solution containing palladium black of the hydrogenation circuit of the anthraquinone process for the production of hydrogen peroxide.
In the anthraquinone process for the production of hydrogen peroxide, also known as the AO process, a reaction support based on one or more 2-alkyl anthraquinones and tetrahydro-2-alkyl anthraquinones is converted with hydrogen into the corresponding hydroquinone form in an organic solvent system in the presence of a catalyst. After the hydrogenation stage, the working solution freed from the catalyst is treated with an oxygen-containing gas in the oxidation stage, the quinone form of the reaction support reforming with elimination of hydrogen peroxide. Finally, hydrogen peroxide is extracted from the oxidized working solution with water and the working solution, i.e. the mixture of the reaction support and solvent or solvent mixture, is returned to the hydrogenation stage. Full particulars of the AO process can be found in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition (1989), Vol. A 13, pages 447-456 and Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition, volume 13, pages 12-38 (which is incorporated by reference).
Where suspension catalysts, such as palladium black for example, are used, the circuit of the hydrogenation stage (see, for example, U.S. Pat. No. 4,428,923 which is incorporated by reference) essentially comprises the actual reactor, a circulation pipe with a circulation pump, means for introducing and distributing the hydrogen, means for introducing working solution from the working solution drying stage following the extraction stage, and means for removing catalyst-free hydrogenated working solution, including a solid/liquid separator. U.S. Pat. No. 4,428,923 discloses a process for the production of hydrogen peroxide by the anthraquinone process which includes a step of the catalyst hydrogenation with hydrogen or a hydrogen containing gas using a working solution having palladium black suspended therein. The process employs a meandering tube system as the reaction space at a temperature up to 100.degree. C. and a pressure up to 15 bar. The hydrogenation is carried out in a loop reactor made of tubes of the same nominal width and which are arranged vertically or horizontally and are connected by tube elbows of the same nominal width at a flow velocity in the tubes of at least 3 m/sec.
The quantitative and--in continuous operation--uninterrupted retention of the generally very finely divided suspension catalyst in the hydrogenation circuit during separation of part of the hydrogenated working solution, which is delivered to the oxidation stage, is a basic prerequisite for a safe and economic process. Accordingly, numerous references are concerned with this particular aspect, the solid/liquid separation being based on filtration using fine-pored filters and catalyst deposited on the filter surface being periodically removed therefrom either completely or in part by pressure reversal and being delivered to the hydrogenation circuit (see, for example CA-PS 1,208,619, GB-PS 959,583 and U.S. Pat. No. 3,433,358).
According to CA-PS 1,208,619, suspended hydrogenation catalysts with a particle spectrum of 75%&gt;1 .mu.m can be separated from a working solution for the production of hydrogen peroxide by using a filter medium of sintered steel particles having a maximum pore diameter of 8 .mu.m. Satisfactory operation of the filter on a continuous basis necessitates backwashing at frequent intervals. Only suspension catalysts with the particle spectrum mentioned above can be separated. This process cannot be used for the separation of a catalyst-free working solution from a working solution containing a noble metal black, for example palladium black having a primary particle spectrum of 5 to 50 nm (see Example 3).
A particular embodiment of the hydrogenation circuit of the anthraquinone process for the production of hydrogen peroxide is disclosed in GB-PS 959,583. Part of the hydrogenated working solution is removed in catalyst-free form from the circulated working solution containing palladium catalyst by means of a filter in a filter housing through which the catalyst-containing working solution flows. The filter is periodically backwashed to free the surface of the filter at least from partly deposited catalyst. No particulars of the filter medium are provided in this document. A major disadvantage of this process is that the volume of catalyst-containing working solution present in the filter housing(s), based on the volume present in the hydrogenation circuit as a whole, is very considerable. Accordingly, this embodiment involves an undesirably high tie-up of capital.
Effective filter media for separating support-free noble metal catalysts, such as palladium black, are known from U.S. Pat. No. 3,433,358 (which is incorporated by reference). The filter media in question are filter candles of carbon material of which the pore diameter may be larger than the diameter of the particles to be separated and which have a wall thickness of at least 10 mm. In this case, also, one or more filter candles are arranged in a correspondingly shaped filter housing. Although this reference seeks to minimize the volume in the filter housings in order to reduce the capital tied up in the catalyst-containing working solution remaining in this unit, narrow limits are imposed by the design of the filter candles to maintain a certain filter area and have already been reached in practice. U.S. Pat. No. 3,433,358 discloses a process for the separation of a suspended finely divided unsupported noble metal catalyst having a primary particle size of between about 0.01 and 1 m.mu. from an organic liquid containing the suspended catalyst. The organic liquid is passed through a porous carbon filter having a maximum pore size of 15 m.mu. to filter off the suspended catalyst. Filter candles of the type mentioned above, which are periodically backwashed in operation, are used in the industrial production of hydrogen peroxide, for example by the process according to U.S. Pat. No. 4,428,923.
In the search to improve the production of hydrogen peroxide by the anthraquinone process in order to reduce costs, there is considerable interest in modifying the apparatus for separating a catalyst-free working solution from the hydrogenation circuit in such a way that the volume of working solution remaining in the solid/liquid separator and the quantity of catalyst present therein can be significantly reduced for the same filtration capacity.
DE-OS 32 45 318 describes a process for continuously carrying out gas/liquid pressure reactions using a suspended catalyst remaining in the reaction zone, in which part of the reaction liquid is removed from the reaction vessel using a microfilter operated on the crossflow principle. The microfilters are equipped with a tube, hose or plate module (polytetrafluoroethylene, polyvinylidene fluoride and sintered metals being mentioned as suitable materials for the module). The microfilter is periodically backwashed by pressure reversal between the retentate side and the permeate side of the microfilter. References to the production of hydrogen peroxide by the anthraquinone process using extremely fine noble metal blacks are as difficult to find in this document as references to the solution to the problem addressed by the present invention as stated above. Thus, the catalyst used by way of example in the process according to DE-OS 32 45 318 has an average particle size of around 1 .mu.m whereas the primary particles of palladium black are between 5 and 50 nm in size. DE-OS 30 40 631 (U.S. Pat. No. 4,414,401 which is incorporated by reference) is based on a process principle similar to DE-OS 32 45 318: among the catalysts mentioned by way of example there is again no reference to the particularly fine-particle noble metal blacks.